std::to_array() function

od C++20

// (1) Lvalue reference argument verison
template<class T, std::size_t N>
constexpr std::array<std::remove_cv_t<T>, N> to_array(T (&a)[N]);

// (2) Rvalue reference argument verison
template<class T, std::size_t N>
constexpr std::array<std::remove_cv_t<T>, N> to_array(T (&&a)[N]);

Creates a std::array from the one dimensional built-in array a. The elements of the std::array are copy-initialized from the corresponding element of a. Copying or moving multidimensional built-in array is not supported.

• (1) For every i in a range [ 0; N ), copy-initializes result's correspond element with a[i]. This overload is ill-formed when std::is_constructible_v<T, T&> is false.
• (2) For every i in a range [ 0; N ), move-initializes result's correspond element with std::move(a[i]). This overload is ill-formed when std::is_move_constructible_v<T> is false.

Both overloads are ill-formed when std::is_array_v<T> is true.

Parameters

• a - the built-in array to be converted to std::array

Type requirements

• T must meet the requirements of CopyConstructible for overload (1)

• T must meet the requirements of MoveConstructible for overload (2)

Return value

• (1)

std::array<std::remove_cv_t<T>, N>{ a[0], ..., a[N - 1] }

• (2)

std::array<std::remove_cv_t<T>, N>{ std::move(a[0]), ..., std::move(a[N - 1]) }

Complexity

Linear in the size of the built-in array.

Notes

There are some occasions where class template argument deduction

of std::array cannot be used while to_array being available:

• to_array can be used when the element type of the std::array is manually specified and the length is deduced, which is preferable when implicit conversion is wanted.
• to_array can copy a string literal, while class template argument deduction constructs a std::array of a single pointer to its first character.

std::to_array<long>({3, 4}); // OK: implicit conversion
// std::array<long>{3, 4};   // error: too few template arguments
std::to_array("foo");        // creates std::array<char, 4>{ 'f', 'o', 'o', '\0' }
std::array{"foo"};           // creates std::array<const char*, 1>{ +"foo" }

Feature testing macro: __cpp_lib_to_array

Possible implementations

Overload (1)

namespace detail {
 
template <class T, std::size_t N, std::size_t... I>
constexpr std::array<std::remove_cv_t<T>, N>
	to_array_impl(T (&a)[N], std::index_sequence<I...>)
{
	return { {a[I]...} };
}
 
}
 
template <class T, std::size_t N>
constexpr std::array<std::remove_cv_t<T>, N> to_array(T (&a)[N])
{
	return detail::to_array_impl(a, std::make_index_sequence<N>{});
}

Overload (2)

namespace detail {
 
template <class T, std::size_t N, std::size_t... I>
constexpr std::array<std::remove_cv_t<T>, N>
	to_array_impl(T (&&a)[N], std::index_sequence<I...>)
{
	return { {std::move(a[I])...} };
}
 
}
 
template <class T, std::size_t N>
constexpr std::array<std::remove_cv_t<T>, N> to_array(T (&&a)[N])
{
	return detail::to_array_impl(std::move(a), std::make_index_sequence<N>{});
}

Example

#include <type_traits>
#include <utility>
#include <array>
#include <memory>

int main()
{
	// copies a string literal
	auto a1 = std::to_array("foo");
	static_assert(a1.size() == 4);
 
	// deduces both element type and length
	auto a2 = std::to_array({ 0, 2, 1, 3 });
	static_assert(std::is_same_v<decltype(a2), std::array<int, 4>>);
 
	// deduces length with element type specified
	// implicit conversion happens
	auto a3 = std::to_array<long>({ 0, 1, 3 });
	static_assert(std::is_same_v<decltype(a3), std::array<long, 3>>);
 
	auto a4 = std::to_array<std::pair<int, float>>(
		{ { 3, .0f }, { 4, .1f }, { 4, .1e23f } });
	static_assert(a4.size() == 3);
 
	// creates a non-copyable std::array
	auto a5 = std::to_array({ std::make_unique<int>(3) });
	static_assert(a5.size() == 1);
 
	// error: copying multidimensional arrays is not supported
	// char s[2][6] = { "nice", "thing" };
	// auto a6 = std::to_array(s);
}

Possible output

(no output)